Generally, a contact hole of a semiconductor device exposes a conductive layer through an overlying interlayer insulating layer. A contact plug filling the contact hole is electrically connected to the conductive layer. The contact plug may be made of doped polysilicon or metal. A conventional method of forming a semiconductor device having a contact hole is discussed with reference to FIG. 1.
Referring to FIG. 1, a device isolation layer 2 is formed on a P-type semiconductor substrate 1 to define an active region. A gate pattern 5 is formed to cross the active region. The gate pattern 5 includes a gate insulating layer 3 and a gate electrode 4 that are sequentially stacked. N-type impurity diffusion layers 6 and 7 are formed in the active region adjacent opposite sides of the gate pattern 5. The N-type impurity diffusion layers 6 and 7 respectively provide a source region and a drain region. The gate pattern 5, the source region 6, and the drain region 7 make up an NMOS transistor.
An interlayer insulating layer 8 is formed on an entire surface of the semiconductor substrate 1 including the transistor. The interlayer insulating layer 8 is patterned to provide contact holes 9 exposing predetermined areas of the source/drain regions 6 and 7. Contact plugs 10 are formed to fill the contact holes 9. Lower sides of the contact plugs 10 are electrically connected to the source/drain regions 6 and 7. The contact plugs 10 can be plugs of phosphorus-doped polycrystalline silicon (P-doped polysilicon). If phosphorus (P) from the contact plugs 10 diffuses to the source/drain regions 6 and 7, the source/drain regions 6 and 7 may increase in doping concentration and/or expanded diffusion regions “F” may be formed at the source/drain regions 6 and 7.
As semiconductor devices are continuously scaled down, contact resistances between the contact plugs 10 and the source/drain regions 6 and 7 may increase. Accordingly, doping concentrations of the contact plugs 10 may be increased to reduce contact resistances. The source/drain regions 6 and 7 may thus be subjected to greater increases in diffusion and resulting increases in doping concentration, so that the expanded diffusion areas “F” may extend even further. As a result, an energy barrier of the drain region 7 may be reduced with increases in a voltage applied to the drain region 7. Stated in other words, a drain-induced barrier lowering (DIBL) effect may be increased. As the DIBL effect increases, a likelihood of punchthrough between the source and drain regions 6 and 7 may also increase.